1. The Field of the Invention
The present invention relates to a method for improving accuracy in a peristaltic pump. More specifically, the present invention relates to a method for reducing variance in the output of a pump due to physical parameters of the tubing.
2. State of the Art
Peristaltic pumps are used in a wide variety of applications due to their ability to deliver a relatively high degree of accuracy in dosing. Peristaltic pumps usually utilize resilient tubing with walls which define a fluid flow lumen having a diameter falling within a specified range of tolerances. The tubing is often made from silicone or similar polymers which allow compression and a relatively quick return to the tubing's original shape. During fluid delivery, the tubing is held in engagement with a pumping member which selectively compresses a portion of the tubing to expel the fluid from that portion of the fluid flow lumen and to drive the fluid through the lumen downstream. Once the pumping member disengages a portion of the tubing, the tubing resumes its original shape and the lumen is filled with additional fluid.
In a linear or curvi-linear peristaltic pump, as shown generally at 10 in FIG. 1A, the piece of tubing 14 may be wrapped or stretched over a plurality of engagement members or pumping fingers 18. The engagement members 18 sequentially extend into the tubing 14 to collapse the tubing and move the fluid downstream. As the engagement members 18 retract, the resilient tubing 14 returns to its normal cross-section, thereby allowing the lumen to fill with fluid which will be moved during the next cycle. The segment of the tubing 14 which is collapsed by the engagement members 18 provides a relatively known volume of fluid in the lumen. Thus, each cycle of the engagement member moves a certain amount of fluid and repeating the cycle multiple times will deliver a desired dose. A more detailed discussion of operation of a curvilinear pump is found in U.S. Pat. No. 6,164,921.
In rotary peristaltic pumps, as shown in FIG. 1B, the tubing 14 is often anchored at opposing ends 14a and 14b to a pump body 20. The anchoring may be accomplished by connectors 24 which connect the resilient tubing 14 to other tubing. (Silicone tubing is often more expensive than other types of tubing so it is economical to use the silicone tubing only for the portion which is disposed on the pump). Between opposing ends 14a and 14b, the tubing is wrapped around a rotor 28 which has a plurality of engagement members or rollers 38, 38a, 38b. As the rollers move around the rotors, the rollers will pinch the tubing and collapse the lumen (as represented by roller 38a). This results in a relatively known quantity of fluid being disposed in the lumen of the tubing section 14c disposed between roller 38a and 38b. As the roller 38a continues to move into the position shown by roller 38, the roller continues collapsing the tubing and forcing fluid in the lumen forward. Thus each third rotation of the rotor 28 delivers a substantially known quantity of fluid downstream. Thus, a user can obtain a desired dose by simply controlling the number of rotations of the rotor. A more detailed explanation of the functioning of a rotary peristaltic pump can be found in U.S. Pat. No. 5,720,721.
While peristaltic pumps are relatively accurate, their accuracy depends in large part on the physical properties of the tubing and physical conditions along the tubing. For example, U.S. Pat. No. 5,720,721 teaches a plurality of pressure sensors 42 which are disposed upstream and downstream from the rotor to detect pressure changes within the tubing which can affect the amount of fluid which is moved with each rotation of the tubing. Bubble detectors 46 can also be used to ensure that the proper amount of liquid is being moved and/or that undesirable air, etc., is not present.
In addition to pressure within the tubing and material within the fluid being pumped, the physical parameters of the tubing itself can have an effect on accuracy. These physical parameters can include the thickness of the wall which defines the lumen, the diameter of the lumen (i.e. interior diameter of the tubing), the length of the tubing, and material properties of the tubing such as the durometer rating (generally related to the compressibility of the tubing) and Young's modulus (generally related to the stretchability of the tubing).
Each of the physical parameters or properties can affect the accuracy of fluid delivery by the pump. For example, if a tube has a larger inner diameter than another tube, it will carry a larger volume of fluid with each cycle of the peristaltic pump due to the larger cross-sectional area. Likewise, variances in the collapsibility of the tubing will affect the amount of solution which is passed through the tubing with each cycle of the pump. Additionally, length is important because stretching the tube will naturally reduce the inner diameter of the tube and reduce the amount of fluid moved with each pump cycle. However, a more stretchable tube can actually deliver a slightly higher amount of fluid with each cycle.
Because the physical parameters of the tubing can have a marked impact on the actual flow volume through the pump, it is common for those making peristaltic pump cassettes or feeding lines for use in medical pumps and other pumps that require a high degree of accuracy to require that tubing stock fall within certain parameters. For example, a company manufacturing an enteral feeding pump cassette or infusion pump cassette, may purchase tubing and require that it be delivered in lengths of 5 inches, ±0.040 inches, having an outer diameter of 0.200 inches, ±0.003 inches, an inner diameter of 0.120 inches, ±0.003 inches, and have a durometer rating of Shore A 50, ±5.
While the variances allowed in the tubing specifications are relatively small, each may have an impact of the volumetric accuracy of the pump. For example, the difference in tubing wall thickness may result in a 2% change in output. Likewise differences in the durometer rating or Young's modulus of the tubing may create a 2% variation in output. When each of the variables is added together, the variations in the tubing can result in a variance of between 5-10% of the total flow output of the tubing.
While it will be appreciated that accuracy of the pump can be increased by further reducing the tolerances in which the tubing must fall, requiring manufacturers to provide tubing more closely meeting the tolerances typically results in a much higher purchase price for the tubing thereby increasing the cost of peristaltic pumping sets for the end user.
Thus, there is a need for a method for improving the accuracy of a peristaltic pump when using tubing falling within a predetermined set of parameters.